1. Field of the Invention
The invention is related to a fixing belt used on a fixing apparatus employed on an electronic image forming apparatus such as a printer and a copying machine, and a method for manufacturing the fixing belt.
2. Description of the Related Art
Various electronic image forming apparatuses such as printers and copying machines using electronic photography processes are normally equipped with an apparatus for fixing a toner image formed on a recording medium such as a recording paper and an overhead projector (OHP) sheet. The thermal roller method has been widely used as a fixing method in such apparatuses.
A thermal roller type fixing apparatus generally has a heat roller with a heating source and a pressure roller that is in contact with the heat roller under pressure. Heat and pressure are applied to a recording medium as it passes through the nip area, or the contact area of the rollers, resultantly causing its toner image fixed on the recording medium.
A large amount of toner is applied on a recording medium in case of a fixing apparatus employed on an electronic image forming apparatus, particularly an image forming apparatus that produces a full-color image. Thus, it is preferable to apply heat to a toner image at a temperature as low as possible for a long time for fixing it by securing the nip width or the length of the nip area in a passing direction as wide or long as possible.
However, the nip width of the conventional thermal roller type fixing apparatus depends on the roller diameter and the thickness of a heat-resistant elastic layer formed on the roller. In order to obtain a large nip width, it is necessary to increase the outer diameter of the thermal roller and/or the thickness of the elastic layer. In case of a thermal roller type fixing apparatus for full-color printing, this causes such problems as bulkiness of the apparatus and degrading of thermal conductance, which in turn results in slower printing speeds.
In the meantime, another fixing method has been proposed where a belt is used. The belt type fixing apparatus includes an endless belt and a pressure roller. The nip area in this case is formed as the belt and the pressure roller contact with each other under pressure. The belt type fixing apparatus has an advantage over the thermal roller type in that it can provide a larger nip width easily, thus resulting in a smaller unit with a faster printing capability.
The endless belt is constructed from a base and an elastic layer made of a material such as rubber. The elastic layer is formed on the base for uniformly squashing unmelted toner on the recording medium. For example, JP-A-6-318,001 disclosed a fixing belt including a base made of electric-casting nickel with a thickness of 45 .mu.m and a silicone rubber layer with a thickness of 50 .mu.m, as well as a fixing belt including a base made of polyimide resin with a thickness of 55 .mu.m and a silicone rubber layer with a thickness of 130 .mu.m.
In the belt type fixing apparatus where pressure and heat are used for fixing, the surface of the recording medium that is holding unfixed toner makes direct contact with the fixing belt, so that a portion of the toner image tends to be transferred to the surface of the rotating member or the fixing belt. Namely, the belt type fixing apparatus has a problem that it has a tendency of causing the offset phenomenon, i.e., the melted toner, which is transferred to the surface of the fixing belt, is transferred back to the trailing edge of the same recording medium or the next medium, thus smearing those recording media. In order to prevent the occurrence of such offset phenomenon, the belt type fixing apparatus has a mechanism that coats a releasing agent on the fixing belt.
The conventional belt type fixing apparatus disclosed by JP-A-6-318,001 has a problem that it is difficult to obtain high quality images with good luster when it is applied to image forming apparatuses intended for full color images. This is due to the fact that deformation produced in the elastic layer of the fixing belt is insufficient for causing the surface of the elastic layer to follow the surface of the recording medium. It is, thus, impossible to squash the entire amount of unmelted toner laid in layers to produce multiple colors.
In order to solve this problem, one alternative solution may seem to be to increase the thickness of the elastic layer of the fixing belt thus increasing the deformation ratio. However, this creates a new problem as follows. As the releasing agent permeates into the elastic layer of the fixing belt, it swells as much as 30%. Since the amount of toner held on the recording medium is large in case of a full color image fixing apparatus, the amount of releasing agent coated on the fixing belt increases as well. As a result, if the elastic layer of the fixing belt is thick, it creates a problem of the elastic layer's swelling in the thickness direction as the releasing agent permeates into it.
For example, in the area where the recording medium passed through, the releasing agent is taken away so that the swelling of the elastic layer reduces, while the elastic layer remains swelled up in the area where the medium did not pass through. It results in creating steps in certain areas of the elastic layer. Consequently, as a large size recording medium is fed immediately following a small size recording medium in case of using a fixing belt having a thick elastic layer, said steps in the elastic layer become more conspicuous and cause fluctuations of the nip area in the width and thickness directions. The matter causes troubles such as density fluctuation, etc.